1. Field of the Invention
The present invention relates to heat transfer tubes which are utilized as vaporization and condensation tubes in apparatus such as heat exchangers and heat pipes.
2. Background Art
Heat transfer tubes made of metals, such as copper, having many straight or helical grooves on the inner surfaces, which can be manufactured by drawing processes, have been known in the past.
These grooves provide the following benefits:
1. When used as condensation tubes, these heat transfer tubes produce improved liquefaction efficiency by increasing the turbulence of the vapors as well as improved nucleation of the liquid phase brought about by the action of the surface irregularities. Furthermore, the surface tension effects on the liquid in the grooves serve to retain the fluid and promote good drainage, leading to increased reflux efficiency.
2. When these tubes are used in vaporizers, the edges of the grooves act as nucleation sites for the bubbles to provide rapid boiling, thus increasing the efficiency of liquid to vapor conversion. Furthermore, the surface tension effects serve to distribute the vaporizing liquid evenly throughout the vaporizer, promoting efficient conversion.
In order to improve the performance of such heat transfer tubes, a heat transfer tube shown in FIG. 33 was proposed in Japanese Patent Application Kokai No. 1-317637. This heat transfer tube comprises many straight or helical grooves 2 and many cuts 3 crossing to the grooves 2 on the inner surface thereof.
This heat transfer tube can be manufactured as follows:
Many primary grooves having a V cross-sectional shape, which will become the cuts 3 later, are formed on the inner surface of a metal tube by drawing a primary plug through the tube. Next, many secondary grooves 2 extending at an angle to the primary grooves are formed by drawing a secondary plug through the tube, and the primary grooves are narrowed by the formed secondary grooves 2, and change into cuts 3.
When this heat transfer tube is used as evaporating tube, many little bubbles of vapor generate from the cuts 3, and boiling of the liquid is accelerated. Therefore evaporation performance is improved in comparison with simple grooved tubes.
Furthermore, since the cuts 3 of this tube are nearly closed, these cuts hold firmly minute bubbles which act as nuclei for the formation of vapor, therefore, good evaporation performance will be maintained for a long time. In contrast, in the heat transfer tube having open grooves instead of the cuts 3, such minute bubbles will flee from the grooves little by little during the operation, and evaporation performance will become gradually lower.
The above mentioned tubes, however, have the following drawbacks:
When the heat transfer tube is used for heat exchanger or the like, it is necessary to enlarge the diameters of ends of the tubes by means of insertion of an enlarging plug in order to connect another tubes to the ends. However, in the tube of FIG. 33, since the tube has many sharp cuts 3 on the inner periphery, a risk arises that the ends of the tube will tear as the plug is inserted. To prevent the cracks at the ends of the tube, the cuts 3 should be made shallow, however such shallow cuts do not offer sufficient effect for promoting evaporation performance.